With the recent development of information communication technology, a variety of wireless communication techniques are being developed. From among them, a WLAN is a technique which enables wireless access to the Internet at home or companies or in a specific service providing area using mobile terminals, such as a Personal Digital Assistant (PDA), a laptop computer, and a Portable Multimedia Player (PMP), on the basis of radio frequency technology.
Since Institute of Electrical and Electronics Engineers (IEEE) 802 (i.e., the standard organization of WLAN technology) has been set up on February, 1980, lots of standardization tasks are being performed.
The initial WLAN technology was able to support the bit rate of 1 to 2 Mbps through frequency hopping, band spreading, and infrared communication using a 2.4 GHz frequency band in accordance with IEEE 802.11, but the recent WLAN technology can support the maximum bit rate of 54 Mbps using Orthogonal Frequency Division Multiplex (OFDM). In addition, in the IEEE 802.11, the standardization of various techniques, such as the improvements of Quality of Service (QoS), the compatibility of Access Point (AP) protocols, security enhancement, radio resource measurement, wireless access vehicular environment for vehicle environments, fast roaming, a mesh network, interworking with an external network, and wireless network management, is put to practical use or being developed.
IEEE 802.11b of the IEEE 802.11 supports a maximum transmission speed of 11 Mbps while using the 2.4 GHz frequency band. IEEE 802.11 a commercialized after the IEEE 802.11 b has reduced the influence of interference as compared with the very complicated 2.4 GHz frequency band by using a 5 GHz frequency band not the 2.4 GHz frequency band and also improved the transmission speed up to a maximum of 54 Mbps using the OFDM technique. However, the IEEE 802.11a is disadvantageous in that the communication distance is shorter than that of the IEEE 802.11 b. Further, IEEE 802.11g implements a maximum communication speed of 54 Mbps using the 2.4 GHz frequency band like the IEEE 802.11b and satisfies backward compatibility. The IEEE 802.11g is being in the spotlight and superior to the IEEE 802.11a even in the communication distance.
Further, as a technique for overcoming limits to the communication speed pointed out as vulnerabilities in the WLAN, there is IEEE 802.11n which has recently been standardized. The IEEE 802.11n has its object to increase the speed and reliability of a network and to expand the operating distance of a wireless network. More particularly, the IEEE 802.11n is based on a Multiple Inputs and Multiple Outputs (MIMO) technique using multiple antennas on both sides of a transmitter and a receiver in order to support a High Throughput (HT) having a data processing speed of 540 Mbps or higher, minimize transmission error, and optimize the data rate. Further, the IEEE 802.11n may use not only a coding method of transmitting several redundant copies in order to increase data reliability, but also an OFDM (Orthogonal Frequency Division Multiplex) method in order to increase the data rate.
With the WLAN being widely spread and applications using the WLAN becoming diverse, a need for a new WLAN system capable of supporting a higher throughput than the data processing speed supported by the IEEE 802.11n is recently gathering strength. A Very High Throughput (VHT) WLAN system is one of IEEE 802.11 WLAN systems which have recently been proposed in order to support a data processing speed of 1 Gbps or higher. The name ‘VHT WLAN system’ is arbitrary. A feasibility test for a system using 8×8 MIMO and a channel bandwidth of 80 MHz or higher so as to provide the throughput of 1 Gbps or higher is in progress.
Here, a WLAN system using a channel bandwidth of 80 MHz or higher must take compatibility with stations in accordance with the IEEE 802.11 a/b/g standards using the channel bandwidth of 20 MHz and stations in accordance with the IEEE 802.11n standard using a channel bandwidth of 20 MHz or 40 MHz into consideration. Hereinafter, a station of a WLAN system using the channel bandwidth of 20 MHz or 40 MHz, including stations supporting the IEEE 802.11 a/b/g/n standards, is referred to a legacy station. For the purpose of compatibility and coexistence with a legacy station, when a VHT station transmits data using a channel bandwidth of 80 MHz or higher, information for the legacy station can be transmitted in the unit of 20 MHz so that the legacy station can recognize the data. However, in case where the same transmission signal of the 20 MHz unit is included, a Peak to Average Power Ratio (PAPR) can be increased on the transmission side. A signal having a high PAPR is problematic in that it increases the cost for implementation because a more complicated power amplifier is required. Accordingly, in a system using the channel bandwidth of 80 MHz, a method of lowering the PAPR of a transmission signal while guaranteeing coexistence with a legacy station must be taken into account.